NF-.kappa.B is a transcription factor that plays a pivotal role in the highly specific pattern of gene expression observed for immune, inflammatory and acute phase response genes, including interleukin 1, interleukin 8, tumor necrosis factor and certain cell adhesion molecules. Like other members of the Rel family of transcriptional activators, NF-.kappa.B is sequestered in an inactive form in the cytoplasm of most cell types. A variety of extracellular stimuli including mitogens, cytokines, antigens, stress inducing agents, UV light and viral proteins initiate a signal transduction pathway that ultimately leads to NF-.kappa.B release and activation.
Important modulators of NF-.kappa.B activation are the inhibitor proteins I.kappa.B.alpha. and I.kappa.B.beta. (referred to herein as I.kappa.B), which associate with (and thereby inactivate) NF-.kappa.B in vivo. Activation and nuclear translocation of NF-.kappa.B occurs following signal-induced phosphorylation of I.kappa.B, which leads to proteolysis via the ubiquitin pathway. For I.kappa.B.alpha., the stimulus-induced phosphorylation at serines 32 and 36 renders the inhibitor a target for ubiquitination at lysines 21 and 22, resulting in degradation. Similarly, phosphorylation of I.kappa.B.beta. at serines 19 and 23 renders the inhibitor a target for ubiquitination at lysine 9. However, neither the site at which I.kappa.Bs are recognized by the ubiquitin system, nor the component(s) of the ubiquitin system mediating I.kappa.B recognition have been identified.
Degradation of a protein via the ubiquitin pathway proceeds by two discrete and successive steps: (a) covalent attachment of multiple ubiquitin molecules to the protein substrate, and (b) degradation of the targeted protein by the 26S proteasome complex. The ubiquitin pathway consists of several components that act in concert and in a hierarchical manner (for reviews, see Ciechanover, Cell 79:13, 1994; Hochstrasser, Curr. Op. Cell. Biol. 7:215, 1995; Jentsch and Schlenker, Cell 82:881, 1995; Deshaies, Trends Cell Biol. 5:428, 1995). One such component, a single E1 enzyme, carries out activation of ubiquitin. Several major species of E2 enzymes have been characterized in mammalian cells, plants, and yeast. E2 enzymes probably bind to the ligase E3 (Reiss and Hersko, J Biol. Chem. 265:3685, 1990; Dohmen et al., Proc. Natl. Acad. Sci. USA 88:7351, 1991) and it appears that each E2 enzyme can act with one or more E3 proteins (Nuber et al., J Biol. Chem. 271:2795, 1996; Orian et al., J Biol. Chem. 270:21707,1995; Stancovski et al., Mol. Cell. Biol. 15:7106, 1995; Gonen et al., J Biol. Chem. 271:302,1996).
Only few E3 enzymes (ubiquitin ligases) have been described. Mammalian E3.alpha. (UBR1 in yeast) and E3.beta. recognize protein substrates via their free N-terminal amino acid residues ("N-end rule"; Varshavsky, Cell 69:725, 1992; Hershko and Ciechanover, Ann. Rev. Biochem. 61:761, 1992). Cdc53 is probably an E3 involved in targeting phosphorylated G1 cyclins (Willems et al., Cell 86:453, 1996). E6-AP is involved in recognition of p53 (Scheffner et al., Cell 75:495, 1993), and a series of unique E6-AP homologous proteins have been identified (Huibregtse et al., Proc. Natl. Acad. Sci. USA 92:2563, 1995): Nedd4 is involved the degradation of the epithelial Na.sup.+ channel (Staub et al, Embo J 15:2371, 1996) and RSP5 (NIP1) is involved in tagging the permeases Gap1 and Fur1 (Hein et al., Mol. Microbiol. 18:77, 1995), whereas Pub1 targets Cdc25 (Nefsky and Beach, EMBO J 15:1301, 1996). Several other E3 enzymes that have been recently isolated appear to be involved in the degradation of c-Fos, a subset of muscle proteins, and in the processing of p105, the NF-.kappa.B precursor (Orian et al., J Biol. Chem. 270:21707, 1995; Stancovski et al., Mol. Cell. Biol. 15:7106, 1995; Gonen et al., J Biol. Chem. 271:302,1996). Thus, it appears that the ligases represent a large, mostly unraveled family of enzymes and, except for the mode of recognition of the "N-end rule" ligases (E3.alpha. and E3.beta.), the recognition motifs of all other known substrates of the ubiquitin system have not been identified.
Accordingly, there is a need in the art for an improved understanding of I.kappa.B degradation via the ubiquitin pathway, and for the identification of modulators of this degradation process for use in treating diseases associated with activation of NF-.kappa.B. The present invention fulfills these needs and further provides other related advantages.